ABSTRACT- The response of belowground ecosystem components to increases in atmospheric CO2 continues to be unpredictable. To determine the effects of this global change on fine root dynamics of the Mojave Desert, minirhizotron tubes were installed to a depth of 1 m beneath the canopies of Larrea tridentata and Ambrosia dumosa at the Nevada Desert FACE Facility. Images were collected every four weeks at depth intervals of 4 cm over 4 years and used to estimate root length density. Specific root length measurements of these species grown under both CO2 treatments were used to calculate biomass density. Fine root standing crop, production, and mortality were then estimated. C and N turnover rates were obtained by multiplying N and C contents by the cumulative root mortality. These calculations were based on the assumption that root images collected were restricted to the particular species being observed. Mean standing biomass, production, and mortality of L. tridentata were significantly lower under the elevated treatment by approximately 45%; however, for A. dumosa, no differences were detected. High values for specific root lengths of L. tridentata (126 g m-3) in the elevated plots relative to the ambient treatment (69 g m-3) were responsible for lower biomass, production, and mortality estimates with high CO2, but no specific root length differences were measured for A. dumosa (93 g m-3 under both treatments). CO2 had no significant effect on belowground N and C turnover rates of A. dumosa, but resulted in a significant reduction of L. tridentata by 40-50%. These reductions in root growth dynamics may indicate a reduced limitation of belowground resources, such as water and nutrients, on L. tridentata growth under high CO2 conditions.